Exposure of the developing embryo to endocrine disrupting chemicals (EDCs) has been proposed to underlie a number of human health problems, including birth defects of genitourinary organs, obesity, decreased fertility and cancer. The most common anomaly of the genitourinary system is hypospadias, a malformation of the external genitalia that is characterized by failure of urethral tube closure and incomplete formation of the prepuce (foreskin) and ventral penis. Affected children can have oversized or multiple urethral openings, and children with severe hypospadias are born with ambiguous genitalia. In the industrialized world, the incidence of hypospadias has risen steadily over the past thirty years and now affects approximately 1 in 125 live male births. Genetic screens of patients with hypospadias have, thus far, failed to identify mutations in candidate genes that can account for this condition, and it has been hypothesized that the high incidence of hypospadias may be due to exposure of the embryo to EDCs in the environment. A number of environmental EDCs have been shown to induce hypospadias in rats (and related defects in wildlife) but little is known about how these factors influence the genetic pathways that operate during development of the external genitalia. Our preliminary studies in mice show that EDCs can induce transient down-regulation of genes that control urethral tube formation, suggesting a mutation-independent mechanism by which these factors can disrupt the genetic program that directs genital development. In this project we propose to integrate mouse developmental genetics and ecotoxicology in order to identify how the gene networks that function during penile development are affected by EDCs. The overarching aim of this proposal is to identify the molecular and cellular events that translate embryonic exposure to an EDC into a structural defect of the genitalia. Identifying the genetic targets of EDCs and determining their functions in the genital tubercle (the embryonic anlagen of the penis and clitoris) is critical if we are to (a) understand the mechanisms by which EDCs perturb normal development, (b) understand why developing genitalia are sensitive to EDCs at specific stages of pregnancy, (c) develop new model systems to test EDC effects on genitourinary cells and tissues, and (d) develop preventative treatments such as supplementation to augment EDC-sensitive pathways.